In Doppler radars, the shift between the transmitted and received frequencies due to the Doppler effect is given by the relationship f.sub.d =2V.sub.r /.lambda. in which V.sub.r is the relative radial velocity of a target with respect to the radar and .lambda. is the wavelength of the transmitted signal.
Usually, the transmitted signal is at a fixed frequency throughout its processing, i.e. during a time interval depending on the resolving power required.
In frequency-agile radars, however, the frequency of the transmitted signal can be varied form one filter cycle to the next in order to foil possible jammers.
The Doppler shift depends on the wavelength of the transmitted signal; if the change in transmitted wavelength is excessive, the corresponding variation in the Doppler shift becomes greater than the bandwidth of an intermediate-frequency filter provided on the reception side of the system. Such a shift in the frequency of the received signal relative to the center of the pass band of this reception-side filter causes a loss of data pertaining to the speed and tracking of the target.
Conventional Doppler tracking radars usually have two oscillators. Thus, a transmission-side oscillator 12 shown in FIG. 1 may be crystal-controlled to supply the purest reference wave possible. A reception-side oscillator 2, feeding a heterodyning signal to first and second mixers A and B, is of the voltage-controlled type responsive to an automatic-frequency-control (AFC) circuit 4 so that the signal received from the target and stepped down to an intermediate-frequency level has a fixed frequency. Circuit 4 is, in turn, controlled by a frequency discriminator 3. A phase loop 6 enables the reception-side oscillator 2 to have the spectral purity of the transmission-side oscillator 1, target tracking being facilitated by introducing into this loop a variable transposition frequency fed to a third mixer C from a so-called Dopper oscillator 5 at a lower frequency which is easier to produce with good spectral purity. Mixer A receives incoming echo signals from an antenna 91 via a duplexer 9 to which outgoing signals are supplied by way of a modulator 8 from oscillator 1; the latter also feeds the mixer B whose output is connected to phase loop 6.
However, during frequency-agile operation such systems do not enable the avoidance of a momentary shift in the mid-frequency of the incoming signal with respect to the center frequency of a narrow-band intermediate-frequency filter 7, which passes the output signal S of mixer A to discriminator 3, since the automatic-frequency-control loop is not fast enough for an instantaneous correction of the shift.